1. Field of the Invention
This invention relates to a liquid crystal display device. More specifically, this invention relates to a liquid crystal display device whose liquid crystals in a display region are driven for display, in which the display region includes both a reflective area for achieving display by reflecting light that makes incident from outside and a transmissive area for achieving display by transmitting light from a rear part.
2. Description of the Related Art
Liquid crystal display devices can be broadly classified into transmissive liquid crystal display devices and reflective liquid crystal display devices. In general, a transmissive liquid crystal display device has a backlight light source, and displays an image by controlling an amount of transmitted light from the backlight light source. A reflective liquid crystal display device has a reflector for reflecting light from outside, and displays an image by utilizing the light reflected by the reflector as a display light source. The reflective liquid crystal display device requires no backlight light source, so that it has an advantage over the transmissive liquid crystal display device in terms of reducing the power consumption, the thickness, and the weight of the device. However, the reflective liquid crystal display device has such a shortcoming that the contrast and visibility become deteriorated under a dark condition, since it uses light in the surroundings as the display light source.
In the meantime, transflective liquid crystal display devices that have advantages of both the transmissive liquid crystal display device and the reflective liquid crystal display device have been put into practical use as displays of portable telephones and mobile terminals. A transflective liquid crystal display device has a transmissive area and a reflective area within a unit pixel. The transmissive area transmits light from a backlight light source, and uses the backlight light source as a display light source. The reflective area has a reflector, and uses external light that is reflected by the reflector as a display light source. With the transflective liquid crystal display device, it is possible to reduce the power consumption by putting out the backlight light source and displaying an image with the reflective areas under a bright condition. Further, it is also possible to display an image even under a dark condition by putting on the backlight light source and displaying an image with the transmissive areas when the surrounding condition turns dark.
Further, as a liquid crystal panel and a liquid crystal display device having a wide range of visible view angles, a transverse electric field mode such as an IPS mode (In-Plane switching) that is generally referred to as a wide view angle liquid crystal panel has been put into practical use. With this IPS-mode liquid crystal panel, liquid crystal molecules are aligned uniaxially by being in parallel to a substrate, and a voltage is applied in parallel to the substrate so as to rotate the liquid crystal molecules while keeping a balanced state with the substrate. That is, the liquid crystal molecules do not rise against the substrate even when the voltage is applied, so that the wide view angles can be obtained theoretically. Further, the IPS-mode liquid crystal display device has a pixel electrode and a common electrode formed on the same substrate, and a lateral electric field is applied to a liquid crystal layer. With the IPS-mode liquid crystal display device, wider view angles than that of a TN-mode liquid crystal display device, for example, can be achieved by displaying an image through rotating the liquid crystal molecules in a direction parallel to the substrate.
When the IPS mode is employed for a related transflective liquid crystal display device, black display and white display are inverted. Thus, if the transmissive area is normally black under a normal driving system, the reflective area becomes normally white.
FIG. 17A and FIG. 17B illustrate schematic sectional views of a unit pixel of the related transflective liquid crystal display device. As shown in FIG. 17A and FIG. 17B, the unit pixel of the transflective liquid crystal display device includes a back-face side substrate 501, a viewer-side substrate 502, and a liquid crystal layer 503 sandwiched between both substrates. The unit pixel includes, in a pixel area, a reflective area 521 for reflecting light from the viewer side, and a transmissive area 522 for transmitting light from the back-face side, and the liquid crystal layer 503 of the reflective area 521 and the transmissive area 522 is driven with a transverse electric field by a voltage that is applied in parallel to the substrate face.
Further, the back-face side substrate 501 and the viewer-side substrate 502 include a first polarizing plate 520 and a second polarizing plate 523, respectively, on the outer side thereof. Furthermore, two kinds of electrodes, i.e. pixel electrodes 511 and common electrodes 512, are formed on the surface of the back-face side substrate 501 on the liquid crystal layer side. In a part of the area where the pixel electrode 511 and the common electrodes 512 are formed, a reflector 515 and an insulating layer 516 are provided between those electrodes and the back-face side substrate 501. The thickness of the liquid crystal layer 503 of the reflective area 521 is a half the thickness of the liquid crystal layer 503 of the transmissive area 522, because of an existence of the insulating layer 516. Further, a backlight 504 functioning as a light source for transmissive display is provided on the outer side (lower side) of the polarizing plate (first polarizing plate) of the back-face side substrate 501.
Further, as illustrated with dotted lines in FIG. 17A and FIG. 17B, the first polarizing plate 520 and the second polarizing plate 523 are arranged in such a manner that the polarizing axes thereof become orthogonal to each other. In the liquid crystal layer 503, the liquid crystal molecules are aligned to face in a direction that is shifted from the polarizing axis (light transmitting axis) of the first polarizing plate 520 by 90 degrees, when no voltage is applied. For example, if the polarizing axis of the first polarizing plate 520 is 0 degree, the polarizing axis of the second polarizing plate 523 is set as 90 degrees, and the major axis direction of the liquid crystal molecules of the liquid crystal layer 503 is set as 90 degrees. In the liquid crystal layer 503, a cell gap in the transmissive area 522 is so adjusted that retardation Δnd (Δn is refractive index anisotropy of the liquid crystal molecules, d is a cell gap of the liquid crystal) becomes λ/2 (λ is a wavelength of light; e.g., λ=550 nm when green light is taken as reference), while a cell gap in the reflective area 521 is so adjusted that the retardation becomes λ/4.
Now, by referring to FIG. 17A, there is described an optical action of the unit pixel of the transflective liquid crystal display device in the above-described structure, when no voltage is applied to the liquid crystal layer 503. First, linear polarized light in the direction of 90 degrees (longitudinal direction) that has passed through the second polarizing plate 523 (referred to as “90-degree linear polarized light” hereinafter) makes incident on the liquid crystal layer 503 of the reflective area 521. In the liquid crystal layer 503, the optical axis of the linear polarized light that makes incident on the liquid crystal layer 503 and the major axis direction of the liquid crystal molecules are consistent. Therefore, the incident light in the state of 90-degree linear polarized light as it is transmits through the liquid crystal layer 503, and it is reflected by the reflector 515. In a case of the linear polarized light, it remains in the state of the linear polarized light even after being reflected. Thus, the reflected light in the state of 90-degree linear polarized light again makes incident on the liquid crystal layer 503. Further, the 90-degree linear polarized light as it is exits from the liquid crystal layer 503 and makes incident on the second polarizing plate 523. The polarizing axis of the second polarizing plate 523 is also 90 degrees, so that the 90-degree linear polarized light transmits through the second polarizing plate 523. Therefore, when there is no voltage applied, the reflective area 521 provides white display.
Next, described is the transmissive area 522 where no voltage is applied. The transverse linear polarized light that has passed through the first polarizing plate 520 makes incident on the liquid crystal layer 503 of the transmissive area 522. In the liquid crystal layer 503, the polarizing direction of the incident light and the major axis direction of the molecules are orthogonal to each other, so that the transverse linear polarized light passes therethrough without changing the polarized state, and makes incident on the second polarizing plate 523. Since the polarizing axis of the second polarizing plate 523 is 90 degrees, the transmitted light cannot passes through the second polarizing plate 523, which results in providing black display.
Next, by referring to FIG. 17B, there is described an optical action of the unit pixel of the transflective liquid crystal display device in the above-described structure, when a voltage is applied to the liquid crystal layer 503. First, linear polarized light in the direction of 90 degrees (longitudinal direction) that has passed through the second polarizing plate 523 makes incident on the liquid crystal layer 503 of the reflective area 521. By applying the voltage, the major axis direction of the liquid crystal in the liquid crystal layer 503 is changed from 0 degree to 45 degrees on the substrate plane. In the liquid crystal layer 503, the polarizing direction of the incident light and the major axis direction of the liquid crystal molecules are shifted from each other by 45 degrees, and the retardation of the liquid crystal is set as λ/4. Therefore, the longitudinal linear polarized light that has made incident on the liquid crystal layer 503 is turned into clockwise circular polarized light, and it makes incident on the reflector 515. The clockwise circular polarized light is reflected by the reflector 515 and turned into counterclockwise circular polarized light. The counterclockwise circular polarized light that has made incident on the liquid crystal layer 503 passes through the liquid crystal layer 503 again, which turns into transverse (0-degree) linear polarized light and makes incident on the second polarizing plate 523. Since the polarizing axis of the second polarizing plate 523 is 90 degrees, the reflected light cannot pass through the reflector 515, which results in providing black display.
Then, the transverse linear polarized light that has passed through the first polarizing plate 520 makes incident on the liquid crystal layer 503 of the transmissive area 522. By applying the voltage, the major axis direction of the liquid crystal molecules in the liquid crystal layer 503 is changed from 0 degree to 45 degrees on the substrate plane. In the liquid crystal layer 503, the polarizing direction of the incident light and the major axis direction of the liquid crystal molecules are shifted from each other by 45 degrees, and the retardation of the liquid crystal is set as λ/2. Therefore, the transverse linear polarized light that has made incident on the liquid crystal layer 503 is turned into longitudinal-direction linear polarized light, and it makes incident on the second polarizing plate 523. Therefore, in the transmissive area 522, the second polarizing plate 523 let through the backlight that has passed through the first polarizing plate 520, which results in providing white display.
With the transflective liquid crystal display device described above, there is such an inconvenience that white display and black display are inverted in the reflective area 521 and the transmissive area 522 in both cases where an electric field is applied to the liquid crystal layer 503 and where it is not applied.
As a measure for this, it is possible to provide consistency in displays of the reflective area and the transmissive area through applying voltages different from each other to the liquid crystal layer in the reflective area and the transmissive area of the transflective liquid crystal display device. For example, in the above-described IPS transflective liquid crystal display device, it is possible to apply different voltages to each of the reflective area and the transmissive area through setting a signal inputted to the common electrode of the reflective area (referred to as a reflective common signal hereinafter) and a signal inputted to the common electrode of the transmissive area (referred to as a transmissive common signal hereinafter) to be in opposite phases from each other.
Now, examples of waveforms of each input signal at the time of black display are shown in FIG. 18. There is a potential difference generated in the reflective area between the pixel electrode and the common electrode in a scanning line selected period, and a voltage is applied to the liquid crystal layer (Vlc of FIG. 18A), which results in providing black display. In the meantime, a signal that is in an inverted phase from that of the reflective common signal is inputted to the transmissive common electrode of the transmissive area. Thus, there is no potential difference generated between the pixel electrode and the common electrode in the scanning line selected period, so that no voltage is applied to the liquid crystal layer (Vlc of FIG. 18B), which also results in providing black display. Further, in order to keep the voltage to be applied to each liquid crystal layer within one frame period, it is necessary for the potential of the pixel electrode to change by following the potential of the common electrode. Since the voltages held in the transmissive area and the reflective area are different, it is necessary to provide a storage capacitance to each of the reflective area and the transmissive area.
Now, changes in the potentials of each electrode at the time of black display will be described. As shown in FIG. 19A, in the reflective area, it is necessary to apply a voltage to the liquid crystal layer at the time of black display. Thus, a potential difference (assumed to be 5V in this case) is generated between the common electrode and the pixel electrode in a scanning line selected period. Thereafter, potentials of the pixel electrode and a reflector 1 become floated in a scanning line non-selected period. Thus, the potentials of the pixel electrode and the reflector 1 are to follow the reflective common signal by synchronizing with it through forming a capacitance with the common electrode. Further, as shown in FIG. 19B, in the transmissive area, no voltage is applied to the liquid crystal layer at the time of black display. Thus, the potentials of the common electrode and the pixel electrode in the scanning line selected period become consistent. Thereafter, in the scanning line non-selected period, the potentials of the pixel electrode and the reflector are to follow the transmissive common signal by synchronizing with it through forming a capacitance with the common electrode.
Further, there is also disclosed a unit pixel structure with which displays of the reflective area and the transmissive area can be made uniform by controlling drives of the IPS transflective liquid crystal display device through providing a storage capacitance, respectively, to the reflective area and the transmissive area of the liquid crystal layer for applying different voltages from each other to the respective areas (Japanese Unexamined Patent Application 2005-191061: Patent Document 1). In Patent Document 1, two TFTs (Thin Film Transistors) corresponding to each of the reflective and transmissive areas as well as a first and a second common electrodes corresponding to each of the reflective and transmissive areas are provided. The reflective area and the transmissive area of the liquid crystal layer are driven by inputting signals that are inverted from each other to the two common electrodes.
Furthermore, there is also disclosed a liquid crystal display device in which: a transistor and a storage capacitance are provided by corresponding to each of a reflective area and a transmissive area; and a transistor Tr and storage capacitance lines CsrL and CstL are provided to each of the reflective area and the transmissive area (Japanese Unexamined Patent Publication 2005-189570: Patent Document 2). In Patent Document 2, the storage capacitance of the reflective area is formed with the pixel electrode and the storage capacitance line CsrL of the reflective area, and the storage capacitance of the transmissive area is formed with the pixel electrode and the storage capacitance line CstL of the transmissive area. Thereby, the storage capacitance is individually formed in each of the reflective area and the transmissive area. With this, different potentials can be applied to the pixel electrodes of the reflective area and the transmissive area. Further, the aperture ratio of the liquid crystal display device can be increased by forming the storage capacitance of the transmissive area in a lower layer of the reflector of the reflective area.
However, with the transverse electric field type transflective liquid crystal display device described above, it is not possible to achieve an excellent display by simply forming the storage capacitances for the reflective area and the transmissive area in the lower layer of the reflector. For example, when the reflective pixel electrode and the transmissive pixel electrode for forming the storage capacitance are disposed in the lower layer of the reflector in the structure of Patent Document 1, capacitance coupling occurs between both pixel electrodes and the reflector. Thus, the potentials of the pixel electrodes are shifted. Therefore, an offset voltage to be described later is applied to the liquid crystal layer, which causes deterioration in the contrast due to a leakage of light. There is also a similar problem raised when the reflective storage capacitance forming part and the transmissive storage capacitance forming part are overlapped with a layer that is made of another conductive substance.